Hi there, live person here!

I'd suggest doing the following:

• Download Malwarebytes (www.malwarebytes.com) and install it, then run a full scan, removing anything it finds.
• Refresh Firefox - reset add-ons and settings
• Restore the default home page

DK1952 said

back to Firefox search

Firefox is a web browser and not a Search engine like Google, Bing, DuckDuckGo, Yahoo etc are.

Neither the Firefox web browser nor the company Mozilla is reverting your Search Engine to Yahoo.

Something else is doing this, like say a Extension you have installed or worst case some form of malware you have on your Windows.

Firefox from mozilla.org does not even come with Yahoo as a search engine option anymore since Fx 57 and later due in part with the legal dispute with Yahoo